Simple and compact laser wavelength locker

ABSTRACT

A wavelength locker for monitoring the wavelength drift of a laser uses a pair of detectors for detecting a power component of the laser beam and a wavelength component of the laser beam, respectively. Various positionings of the power detector and/or variations to the collimating lens provide a compact arrangement with fewer components.

FIELD OF THE INVENTION

[0001] Embodiments of the present invention are directed to wavelengthlockers and, more particularly, embodiments of the present invention aredirected to more compact wavelength lockers conserving valuable packagespace.

BACKGROUND INFORMATION

[0002] Wavelength division multiplexing (WDM) is a technique used totransmit multiple channels of data simultaneously over the same opticfiber. At a transmitter end, different data channels are modulated usinglight having different wavelengths or, colors for each channel. Thefiber can simultaneously carry multiple channels in this manner. At areceiving end, these channels are easily separated prior to demodulationusing appropriate wavelength filtering techniques.

[0003] The need to transmit greater amounts of data over a fiber has ledto so-called Dense Wavelength Division Multiplexing (DWDM). DWDMinvolves packing additional channels into a given bandwidth space. Theresultant narrower spacing between adjacent channels carried by a fiberin DWDM systems demands precision wavelength accuracy from thetransmitting laser diodes.

[0004] Unfortunately, as laser diodes age, they are known to exhibit awavelength drift of up to 0.15 nm from their set frequency over about afifteen year period. This period is well within the expected servicelife of modern laser diodes. Hence, this wavelength drift isunacceptable as a given channel may drift and interfere with adjacentchannels causing cross talk. To remedy this situation most lasertransmitters use what is commonly referred to in the art as a wavelengthlocker to measure drift frequency vs. set frequency. This informationcan be fed back to a controller to adjust various parameters, such astemperature or drive current, of the laser diode to compensate for theeffects of aging and keep the diode laser operating at its setfrequency. Most laser transmitters with an integrated wavelength lockeruse either an etalon or thin film filter to measure the laser wavelengthvariation.

[0005]FIGS. 1A and 1B show a type of conventional wavelength lockerconfiguration. A laser 6 produces a laser beam centered about a setfrequency or wavelength. The laser 6 emits a light beam from both afront facet 15 and a back facet 13. The actual modulated light carryingthe data channel emerges from the front facet 16, which is coupled to anoptical fiber (not shown). The beam 12 that emerges from the back facet13 is used for monitoring purposes since it has the same wavelength asthe beam emerging from the front facet 15. The monitored beam 12 passesthrough a lens 8. A beam splitter 10 splits a monitored beam 12 into twobeams. The first beam 14 passes through the splitter 10 and is receivedby a first detector 16, hereinafter referred to as the power monitordetector 16. The second beam 20 is deflected and passes through awavelength filter (etalon) 22 after which it is received by a seconddetector 24, hereinafter referred to as the filter detector 24.

[0006] In operation, the detectors 16 and 24, which may be for example,photodiode or optoelectrical detectors, output an electric signal basedon the optical input of the received beam. The first detector 16receives the first beam 14 and outputs a signal that is a function ofthe monitored beam's 12 power. The second detector 24 receives thesecond beam 20 and outputs a signal that is a function of both themonitored beam's 12 power as well as its wavelength. Thus, bymathematically operating on these signals as output by the detectors, 16and 24, the wavelength of the monitored laser beam 12 can be determinedand compared to the set frequency to determine any wavelength drift ofthe laser's 6 output.

[0007] The above configuration includes a beam splitter 10 as well as afilter 22 and second detector 24, positioned perpendicular to theoptical axis of the monitored beam 12. Thus, this arrangement takes upan undesirably large amount of space in an optical device package.

[0008]FIG. 2 shows an alternate wavelength locker configuration thatuses a “stacked” arrangement of detectors. As shown, the filter detector26 and the power monitor detector 27 are stacked one on top of the otherwith a filter 28 placed in front of the filter detector 26. A collimatedbeam 29 strikes both of the detectors, 26 and 27 with the lower portionof the beam 29 first passing through the filter 28 prior to striking thefilter detector 26. Unfortunately, in this configuration the centerportion of the collimated beam 29 where the power of the beam is thehighest is not used. Thus, this configuration is not as sensitive todetect small changes in the beam as is desired.

[0009] Since optoelectronics packaging is one of the most difficult andcostly operations in the manufacturing process, designers are alwaysstriving for simpler more compact cost effective arrangements andsolutions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following is a brief description of the drawings, whereinlike numerals indicate like elements throughout:

[0011]FIG. 1A is a plan view of conventional configuration for awavelength locker;

[0012]FIG. 1B is a block diagram of the wavelength locker shown in FIG.1A;

[0013]FIG. 2 is a block diagram of a conventional stacked detectorwavelength locker;

[0014]FIG. 3 is a block diagram of a wavelength locker according to oneembodiment of the invention;

[0015]FIG. 4 is a plan view of wavelength locker according to oneembodiment of the invention;

[0016]FIG. 5 is a diagram plotting the filter response for variousplacements of the power monitor detector;

[0017]FIG. 6 is a block diagram of a wavelength locker according toanother embodiment of the invention;

[0018] FIGS. 7A-7B show plan views of yet another embodiment of theinvention using the GRIN lens as both a collimator and a beam splitter;and

[0019]FIG. 8 is a ray tracing diagram showing the operation of the GRINlens of FIGS. 7A-7B.

DETAILED DESCRIPTION

[0020] One embodiment of the present invention is shown in FIG. 3. Here,a back facet 30 of a laser diode 32 outputs a monitored beam 34. Themonitored beam 34 passes through a lens 36 to produce a collimated beam38. The collimated beam 38 passes through a filter (etalon) 40 andthereafter the now collimated, filtered beam 42 falls on a filterdetector 46 which outputs a signal indicating the power of the beam 34as well as the wavelength of the beam being output by the laser diode32.

[0021] Unlike the conventional examples shown in FIGS. 1A and 1B, nobeam splitter is used. Instead, the second detector 48 is placeddirectly in the path of the monitored beam in front of the lens 36. Thesignals output by the detectors, 46 and 48, can be mathematicallyoperated on to determine the wavelength of the monitored beam 34. Twocases for possible placement of the second detector are shown in FIG. 3.In the first case (case 1), the power monitor detector 48 is centered inthe path of the monitored beam 34 about 10 μm behind the laser 32. Inthe second case (case 2) the power monitor detector 48′ is placed about30 μm behind the laser and offset to one side by about 10 μm.

[0022]FIG. 4 shows a set up for testing the impact of a detector betweenthe lens 36 and the laser 32 on the etalon 40 to measure the etalonresponse. As shown, the set up comprises a laser diode 32 mounted on asubstrate 31. The power monitor detector 48 is also mounted on thesubstrate behind the laser diode 32. A collimating lens 32 collimatesthe light from the laser 32 which is then filtered by filter 40 and isdetected by the filter detector 46.

[0023] Measurements were taken with the power monitor photodiode 48placed at two different locations as discussed above. For the firstmeasurement, the power monitor photodiode 40 was placed approximately 10μm behind the laser diode 32. For the second measurement, the powermonitor photodiode 40 was placed approximately 30 μm behind and 10 μm tothe side of the laser diode 32. In both cases, sufficient light wascollected by filter detector 46 for the wavelength locker to operatewithin acceptable specifications. For the disclosed embodiments aminimum signal strength of 20 μA output by the filter detector 36 isrequired for effective wavelength locking. In the first case, the lightcollected produced a 136 μA signal output from the filter detector 46.In the second case, a 72 μA signal was produced from the collected lightby the filter detector 46. Both, well within the acceptable range.

[0024] In addition to signal strength, the extinction ratio (ER) is alsoa factor that needs to be considered. When positioning the power monitordetector 40 in the direct path of the monitored laser beam it blockssome of the light that would otherwise pass through the etalon 40 andreach the filter detector 46. The extinction ratio (ER) is a measure ofthe effectiveness of the etalon filter for wavelength locking. Theextinction ratio is defined as:

[0025] ER=(Maximum filter detector current)/(minimum filter detectorcurrent). The minimum ER specification for the disclosed embodiments is3 dB.

[0026] As shown in FIG. 5, without the power monitor detector 48partially blocking the path of the laser, the measured ER was 4.9 dB.With the detector 10 μm behind the laser, the measured ER was 4.3 dB.Finally, with the detector 30 μm behind the laser 32 and 10 μm to theside of the laser 32 a higher ER of 5.3 dB was measured. Thesemeasurements are shown in FIG. 5 which again demonstrates that asufficient ER measurement can be obtained. In particular, it is notedthat there is no appreciable change in etalon response as the powermonitor detector 48 is repositioned between the etalon 40 and the laser32.

[0027] This embodiment of the invention eliminates the need for a beamsplitter as well as reduces the overall footprint of the wavelengthlocker saving package space. Of course, the examples offered show thepower monitor detector 48 in two alternate positions; however, it isunderstood by those skilled in the art that the power detector 48 couldbe anywhere within the area of the beam 34 so long as sufficient lightcan be gathered by the detectors 40 and 46. For example, the powerdetector may be positioned 5-15 μm behind the laser 32 and 20-40 μm tothe side of the laser 32.

[0028]FIG. 6 shows another embodiment of the invention that uses a lenshaving an angled, polished face to split the monitored beam between thetwo detectors. As shown, the back facet 60 of a laser diode 62 outputs amonitored beam 64 which is collimated through a micro-gradient index(GRIN) lens 65. The end face 66 of the GRIN 65 is angled at 45 degreesand is coated with a broadband partially reflective coating. Of courseother angles may be appropriate such as in a range between 30-60degrees. The GRIN lens 65 used in this fashion permits the use of asingle element as both a collimator and a splitter.

[0029] The splitting ratio can be selected by the appropriate selectionof the coating material. For example, a coating may be selected toprovide for 30% transmission and 70% reflection of passing light. A thinfilm filter 67 filters the reflected beam. The power monitor detector 68gives a signal (signal 1) proportional to power only and the filterdetector 69 gives a signal (signal 2) that is a function of wavelengthand power. As before, by mathematically operating on these two signals,as with controller 61, the wavelength of the monitored beam 64 can bedetermined.

[0030] Alternatively, the filter 67 can be omitted and instead, a thinfilm filter 65 can be applied directly on the GRIN end face 66. In thiscase, both detectors, 67 and 68, produce a signal having a function ofwavelength since filtered light reaches both detectors. In this case,the sum of the two signals can be used to monitor the power of thelaser. Further, in this alternate arrangement, the difference of the twodetector signals has twice the slope vs. wavelength compare the casewhen the filter 67 is used, effectively enhancing the wavelength lockersensitivity.

[0031] FIGS. 7A-B show yet another embodiment of the present inventionsimilar to the embodiment shown in FIG. 4. A laser 70 is mounted on asub-mount 71 on a substrate 72. A monitored beam from the back facet ofthe laser 70 is collimated with a GRIN lens 73. A thin film reflectivecoating filter 74 is placed on the far end of the GRIN lens 73 thatallows a portion of the monitored beam to pass through. As shown in FIG.7A the portion of the monitored beam that passes through is filtered bya filter 75 and then passes to the filter detector 76. In FIG. 7B, thefilter 75 is replaced by a thin film filter 75′ also coating the GRINlens 73 However, unlike the previous embodiments, the power detector 77is placed adjacent to the laser 70 since a second portion of themonitored light is reflected back through the GRIN lens by the thin filmreflective coating 74.

[0032] This is better shown in FIG. 8. The GRIN lens 73 collects themonitored light 78 from the laser diode 70. The GRIN lens 73 collimatesthe light. The partially reflective coating 74 applied on the end faceof the GRIN reflects a portion of the light back 79 while allowinganother portion of the light to pass 80. The light reflected back isfocused on the power detector 77 located near the laser 77. In thisconfiguration the GRIN lens 73 acts as both a lens and beam splitter.This wavelength locker can be tuned simply by moving the lens 73 intranslation or rotate the assembly containing the filter 75.

[0033] Several embodiments of the present invention are specificallyillustrated and/or described herein. However, it will be appreciatedthat modifications and variations of the present invention are coveredby the above teachings and within the purview of the appended claimswithout departing from the spirit and intended scope of the invention.

What is claimed is:
 1. A wavelength locker, comprising: a lens tocollimate a monitored beam from a light source to produce a collimatedbeam; a filter to filter the collimated beam to output a filtered beam;a first detector to detect the filtered beam; and a second detectorpositioned in a path of the monitored beam between the light source andsaid lens.
 2. The wavelength locker as recited in claim 1, wherein saidfirst detector produces a first signal which is a function of themonitored beam power and the second detector produces a second signalwhich is a function of the monitored beam wavelength.
 3. The wavelengthlocker as recited in claim 1 wherein said second detector is positionedapproximately 5-15 μm in front of the light source.
 4. The wavelengthlocker as recited in claim 1 wherein said second detector is positionedapproximately 5-15 μm in front of the light source and approximately20-40 μm to one side of the light source.
 5. A method for operating awavelength locker, comprising: collimating a monitored beam from asource to produce a collimated beam; filtering the collimated beam andoutputting a filtered beam; detecting the filtered beam and producing afirst signal as a function of monitored beam wavelength; and detectingthe monitored beam emerging from the light source prior to collimatingand producing a second signal as a function of the monitored beam power.6. A method for operating a wavelength locker are recited in claim 5,further comprising: operating on said first signal and said secondsignal to determine a wavelength of the monitored beam.
 7. A wavelengthlocker comprising: a first detector; a second detector; a collimatinglens having first end to receive a monitored beam and a second endhaving an angled polished face to split said monitored beam between saidfirst detector and said second detector; and a filter between saidcollimating lens and at least one of said first detector and said seconddetector.
 8. The wavelength locker are recited in claim 7 wherein saidangled polished face is a 45 degree angle to an optical axis of saidcollimating lens.
 9. The wavelength locker as recited in claim 8 whereinsaid first detector is positioned along the optical axis of saidcollimating lens and said second detector is positioned perpendicular tosaid optical axis of said collimating lens.
 10. The wavelength locker asrecited in claim 9 wherein said filter is positioned between saidcollimating lens and said second detector.
 11. The wavelength locker asrecited in claim 9 wherein said filter comprises a thin film filterapplied directly to the polished face of said collimating lens.
 12. Amethod of monitoring a wavelength of a beam, comprising: providing alens having an angled polished face; collimating a monitored beam withsaid lens; splitting said collimated beam into a first beam and a secondbeam with said angled polished face of said lens; wavelength filteringat least one of said first beam and said second beam; detecting saidfirst beam with a first detector to output a first signal detecting saidsecond beam with a second detector to output a second signal; and usingsaid first signal and said second signal to determine a wavelength ofsaid monitored beam.
 13. The method of monitoring a wavelength of a beamas recited in claim 12 wherein said angled polished face comprises a 45degree angle to an optical axis of said lens.
 14. The method ofmonitoring a wavelength of a beam are recited in claim 12 furthercomprising: placing a wavelength filter between said second detector andsaid lens.
 15. The method of monitoring a wavelength of a beam asrecited in claim 12 further comprising: placing a thin film wavelengthfilter directly in said angled polished face of said lens.
 16. Awavelength locker comprising: a lens to collimate a monitored beam froma light source; a partially reflective coating on one end of said lensto allow a first portion of said monitored beam to pass and to reflectback a second portion of said monitored beam through said lens; a filterto filter said first portion of said monitored beam; a first detector todetect the filtered first portion of said monitored beam; and a seconddetector positioned adjacent to said light source to detect said secondportion of said monitored beam reflected back through said lens.
 17. Thewavelength locker as recited in claim 16 wherein said lens is a gradientindex (GRIN) lens.
 18. The wavelength locker as recited in claim 16wherein said partially reflective coating reflects approximately 70% ofsaid monitored beam back through said lens.
 19. A method of monitoring awavelength of a monitored beam comprising: collimating a monitored beamwith a lens; allowing a first portion of said monitored beam to passthrough the lens wavelength filtering said first portion of saidmonitored beam; detecting said first portion of said monitored beam;reflecting back a second portion of said monitored beam through saidlens to produce a first signal; detecting said second portion of saidmonitored beam to produce a second signal; and using said first signaland said second signal to determine a wavelength of said monitored beam.20. The method of monitoring a wavelength of a monitored beam as recitedin claim 19 wherein said second portion of said monitored beam comprisesapproximately 70% of said monitored beam.